1. Field of the Invention
The present invention generally relates to a semiconductor device, and to a method of designing the semiconductor device. More particularly, the present invention relates to a semiconductor device equipped with a sense amplifier which becomes operative when receiving a read enable signal, and to a method of designing the semiconductor device.
2. Description of the Background Art
There has been known a semiconductor device, e.g., a memory IC, equipped with a sense amplifier which becomes operative when receiving a read enable signal.
FIG. 9 is a circuit diagram showing a sense amplifier 10 and other elements provided for the existing semiconductor device. The sense amplifier 10 has a Data terminal 12 and a /Data terminal 14. The Data terminal 12 and the /Data terminal 14 are connected to an unillustrated Data line and an unillustrated /Data line, respectively. The Data line and the /Data line are transmission lines each of which receives Data or /Data signal from a corresponding memory cell when the address of the memory cell is designated.
The sense amplifier 10 has an output terminal 16 and an enable terminal 18. The sense amplifier is a differential amplifier which amplifies a voltage across the Data terminal 12 and the /Data terminal 14 and outputs the thus-amplified voltage from the output terminal 16 when receiving a read enable signal at the enable terminal 18. The enable terminal 18 of the sense amplifier 10 is connected to a delay circuit 20 formed from a plurality of inverter circuits connected in series.
FIGS. 10A and 10B are timing charts for explaining the operation of the delay circuit 20. FIG. 10A shows a waveform of the read enable signal supplied to the delay circuit 20 from an internal circuit of the semiconductor device. In contrast, FIG. 10B shows a waveform appearing at a node A shown in FIG. 9, i.e., the enable terminal 18 of the sense amplifier 10.
As shown in FIGS. 10A and 10B the delay circuit 20 supplies a read enable signal produced by the internal circuit of the semiconductor device to the enable terminal 18 of the sense amplifier 10 after a lapse of a predetermined delay time T. Accordingly, the sense amplifier 10 commences to amplify the Data signal when the predetermined delay time T has elapsed after the internal circuit of the semiconductor device has changed the enable signal from a low state to a high state.
In a semiconductor device, a certain length of time is required for the Data signal issued from the memory cell to reach the sense amplifier 10 after designation of an address of the memory cell from which data are to be output. The existing semiconductor device reliably performs the amplifying function consuming a small amount of power when the delay time T of the delay circuit 20 matches a propagation time of the Data signal. Therefore, there is desired that the delay circuit 20 is provided so as to meet the aforementioned conditions.
As shown in FIG. 9, the existing semiconductor device has a backup delay circuit 22 provided in a circuit board in order to satisfy the aforementioned requirements. If the sense amplifier 10 is not enabled at a desired timing; namely, if the delay time T generated by the delay circuit 20 is not a desired time, the circuit for propagating the read enable signal is changed to the backup delay circuit 22 by changing a mask used for forming an aluminum wiring layer or a through hole. In the existing semiconductor device, the desired delay time T is ensured by changing the circuit configuration by trial and error under the previously-described method.
However, according to the existing method, it is required to make different prototype circuits by replacing masks over and over again until the configuration of the delay circuit is determined. Consequently, a large cost and much time are required to determine the circuit configuration when the existing method is used in a designing phase of the semiconductor device.